Blue-to-Green Emitting Neutral Ir(III) Complexes Bearing Pentafluorosulfanyl Groups: A Combined Experimental and Theoretical Study

[Image: see text] A structure–property relationship study of neutral heteroleptic (1 and 2, [Ir(C(∧)N)(2)(L(∧)X)]) and homoleptic (3 and 4, fac-[Ir(C(∧)N)(3)]) Ir(III) complexes (where L(∧)X = anionic 2,2,6,6-tetramethylheptane-3,5-dionato-κO(3),κO(6) (thd) and C(∧)N = a cyclometalating ligand bearing a pentafluorosulfanyl (−SF(5)) electron-withdrawing group (EWG) at the C(4) (HL1) and C(3) (HL2) positions of the phenyl moiety) is presented. These complexes have been fully structurally characterized, including by single-crystal X-ray diffraction, and their electrochemical and optical properties have also been extensively studied. While complexes 1 ([Ir(L1)(2)(thd)]), 3 (Ir(L1)(3)), and 4 (Ir(L2)(3)) exhibit irreversible first reduction waves based on the pentafluorosulfanyl substituent in the range of −1.71 to −1.88 V (vs SCE), complex 2 ([Ir(L2)(2)(thd)]) exhibits a quasi-reversible pyridine(C(∧)N)-based first reduction wave that is anodically shifted at −1.38 V. The metal + C(∧)N ligand oxidation waves are all quasi-reversible in the range of 1.08–1.54 V (vs SCE). The optical gap, determined from the lowest energy absorption maxima, decreases from 4 to 2 to 3 to 1, and this trend is consistent with the Hammett behavior (σ(m)/σ(p) with respect to the metal–carbon bond) of the −SF(5) EWG. In degassed acetonitrile, for complexes 2–4, introduction of the −SF(5) group produced a blue-shifted emission (λ(em) 484–506 nm) in comparison to reference complexes [Ir(ppy)(2)(acac)] (R1, where acac = acetylacetonato) (λ(em) 528 nm in MeCN), [Ir(CF(3)-ppy) (acac)] (R3, where CF(3)-ppyH = 2-(4-(trifluoromethyl)phenyl)pyridine) (λ(em) 522 nm in DCM), and [Ir(CF(3)-ppy)(3)] (R8) (λ(em) 507 nm in MeCN). The emission of complex 1, in contrast, was modestly red shifted (λ(em) 534 nm). Complexes 2 and 4, where the −SF(5) EWG is substituted para to the Ir–C(C(∧)N) bond, are efficient phosphorescent emitters, with high photoluminescence quantum yields (Φ(PL) = 58–79% in degassed MeCN solution) and microsecond emission lifetimes (τ(ε) = 1.35–3.02 μs). Theoretical and experimental observations point toward excited states that are principally ligand centered ((3)LC) in nature, but with a minor metal-to-ligand charge-transfer ((3)MLCT) transition component, as a function of the regiochemistry of the pentafluorosulfanyl group. The (3)LC character is predominant over the mixed (3)CT character for complexes 1, 2, and 4, while in complex 3, there is exclusive (3)LC character as demonstrated by unrestricted density functional theory (DFT) calculations. The short emission lifetimes and reasonable Φ(PL) values in doped thin film (5 wt % in PMMA), particularly for 4, suggest that these neutral complexes would be attractive candidate emitters in organic light-emitting diodes.